1. Field of the Invention
The present invention relates to a water supply system for a fuel cell.
2. Description of the Prior Art
A known type of fuel cell utilizes a polymer electron membrane (PEM) having an anode side over which a gas flow of a fuel gas such as hydrogen passes and a cathode side over which a gas flow of an oxidant such as air passes. When an electric load is connected between the anode and cathode, hydrogen ions migrate through the membrane to generate electricity in an electrochemical reaction which drives the load. A byproduct of the electrochemical reaction is water. In order to improve the efficiency of the electrochemical reaction, the gas flows are both pressurized and humidified. An additional reason for humidifying the gases is to ensure that the membrane does not dry out since this not only leads to a reduction in the efficiency of the migration of hydrogen ions across the membrane, but can damage and reduce the service life of the membrane. One method of humidifying the gases is to inject water in the form of an atomized spray into the gas stream.
It is also necessary for the correct operation of the fuel cell to heat or cool the gases at various stages in the flow of the two gas streams to and from the fuel cell. One method of achieving this is to spray water, preferably in an atomized form, into the gas stream. It is therefore necessary to provide a multiplicity of water injection stations in the fuel cell apparatus each of which has different output requirements in terms of the volume of water which has to be injected. For the efficiency of atomization, it is necessary for the pressure of the water to be constant for a given water injector. For fuel cells operating under steady-state conditions, it is relatively easy to set each water injector with the appropriate characteristics for its particular application in the fuel cell. However, under transient conditions, such as occur when fuel cells are used in automobiles, it is not possible to set each water injector since the pressure and flow of the two gas streams and the amount of heating or cooling that the gas streams require changes rapidly depending upon the power output demands which occur under normal driving conditions.
It has been proposed for such applications to use a separate metering pump for each water injector to adjust the output of each water injector selectively depending upon the particular requirements of the parameter which it is influencing at any given time. The use of a multiplicity of separate metering pumps is costly, significant demands are made for space in the vehicle, the vehicle weight is increased, and the pumps take a relatively large amount of power to drive them.
The present invention seeks to provide apparatus for supplying the necessary water injection sprays for a fuel cell in a more cost effective, and space and power efficient manner.